TORONTO — Saturday was a milestone day in the career of Red Sox pitcher John Lackey, who was making his first start in more than 18 months following reconstructive elbow surgery.

The righthander pitched well, too, throwing the best fastball he has in years. It zipped into the glove of catcher David Ross at 94 miles per hour, and Lackey piled up strikeouts against the Toronto Blue Jays.

Then, in the fifth inning, Lackey released a pitch and clutched his arm before staggering off the mound. Teammates rushed to his side as he doubled over in pain. Lackey looked up at the gray roof of the Rogers Centre and muttered a curse.

All Lackey could think at the time was that he had suffered another ligament tear and would need surgery again. But the initial diagnosis was a strain of the biceps, not an elbow injury.

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“It’s sore. But hopefully not too serious,” a downcast Lackey said after the Sox were beaten, 5-0.

Lackey, 34, had not pitched in a major league game since Sept. 25, 2011. With Toronto leading, 2-0, in the fifth, Lackey put away Emilio Bonifacio for his eighth strikeout and was looking to do the same with Jose Reyes.

But the 2-and-2 pitch he threw went awry, nearly hitting Reyes in the front foot. Ross, who had called for a cut fastball on the outside part of the plate, knew something was wrong.

“I turned around and he was holding his arm. Hopefully he’s OK,” Ross said. “It stinks. I think everybody is worried about him and will say a prayer for him. He’s such a good teammate, first of all. To come back and how well he was throwing the ball, I was excited for him.”

Lackey, who felt a twinge on the previous pitch and thought nothing of it, described a cramping sensation that was akin to a hamstring injury. That feeling abated once he was treated with ice by the team trainers.

“It grabbed me pretty good on the last pitch,” Lackey said. “My hand didn’t go numb or anything like that like I’ve had with the elbow stuff. It was pretty much right in the belly of the bicep.”

According to manager John Farrell, the initial tests were encouraging.

“The strength test, at least in the training room, showed to be positive in terms of full strength,” he said. “Any time a pitcher walks off in the middle of an at-bat, there’s always concern. We’ve just got to get all the information we can tomorrow. But we can’t rule anything out and that’s why the exam will take place.”

Lackey told teammates he didn’t believe the injury was too serious.

“The tests they ran on me here today looked pretty good. So hopefully get a picture of it tomorrow and we can get after it and hopefully it’s nothing too crazy,” he said. “It got better when I got inside. It kind of loosened up.”

Farrell did not speculate on how the Sox would replace Lackey in the rotation if that is needed. Alfredo Aceves finished the game and allowed three runs on three hits over 3⅔ innings. He would be the leading candidate.

Until the point of the injury, Lackey threw 51 of his 76 pitches for strikes. His only mistake came in the fourth inning when J.P. Arencibia hit a two-run home run.

Said Farrell: “He looked sharp. He had good action to his secondary stuff. The velocity was probably the best we’ve seen to date. He was free and easy. He threw all his pitches for strikes.”

The Red Sox, meanwhile, were held to two hits in the game. The first was a leadoff double in the first inning by Jacoby Ellsbury. The next did not come until Dustin Pedroia reached on an infield single with two outs in the ninth.

The Sox put only three other runners on base, all via walks. Designated hitter Jonny Gomes said the injury took something out of the Sox.

“The one thing you need in this game is good health,” he said. “Your heart skips a beat.”

Toronto lefthander J.A. Happ and three relievers combined to strike out 11. The Red Sox were 0 for 5 with runners in scoring position and are 2 of 24 the last two games.

The Sox had scored 23 runs on 47 hits in their first four games.

Farrell slumped in his office chair after meeting with reporters.

“That was tough to see,” he said. “John was pitching so well. Hopefully it’s not something bad.”